How to make the background all black

Hi guy, I am trying to figure out how to change the background of the magic mirror to a very deep black(darkest black ) and I am kind of course fused because idk how to do that. I know is some css background color I have to change but which one ? .

For the record my background is all black, but it’s not black like how I want it. When I turn off all modules, you can actually see that the monitor is on and I don’t want that.